In this photo, Margaret is filling what is known as the "running tank" with buffer. If TBE was used to make the gel, it should also be used as the running buffer, or strange things could happen and interpretation of results becomes difficult.
This tank has a raised stage in the middle on which the gel rests, and deeper reservoirs on the sides to hold buffer. The gel will be completely submerged as it is run, assuring that the entire gel is subjected to equal electric current. Notice the two metal pins marked by the arrows. These are the connection plugs for the power supply cables. Each plug leads to a thin wire at one end of the tank or the other. Because one lead is positive and one negative, there will be a strong electrical current flowing through the tank when the electrodes are immersed. It is this current that moves the DNA through the gel. Is DNA positively or negatively charged? Toward which electrode do you think it will migrate? Do you now understand why Boric acid or Acetic acid is included in the electrophoresis buffer?
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